(a) Field of the Invention
The present invention relates to a multi-component composite membrane and a method for preparing the same, and more particularly to a multi-component composite membrane comprising a support layer and an active layer having a structure as dense as the conventional active layers, which is capable of having pores formed thereon according to process conditions and with improved permeability due to the pores of a controllable size, the composite membrane having characteristics of an active layer, and with an interfacial adhesion strength between the support layer and the active layer strengthened by ion beams irradiation, and a method of preparing the same.
(b) Description of the Related Art
Many types of membranes are currently in use, such as microfiltration membranes, ultrafiltration membranes, gas separation membranes, pervaporation membranes, and reverse osmosis membranes.
The present invention relates to a microfiltration membrane, and in particular to a separator comprising polyolefins such as polyethylene and polypropylene, for a rechargeable lithium ion battery.
As one of the polyolefins, when high crystalline polypropylene (HCPP) is used for a separator of the present invention, it is expected that the permeability of the separator will increase. The crystallinity of common polypropylenes is less than 50%, but the crystallinity of HCPP is greater than 50% and it is highly isotactic, so that density, melting point, heat of fusion, and crystallization temperature are high, and characteristics such as rigidity, heat-resistance, impact strength, scratch-resistance, and dimensional stability are excellent.
A composite membrane is generally prepared by interfacial polymerization, modification of membranes, and dip coating. Dip coating is widely used in order to prepare the composite membrane, by using a microporous membrane such as a microfiltration membrane or an ultrafiltration membrane as a support layer, coating the microporous membrane with a solution of a material used as an active layer, and drying the coated membrane. The composite membrane prepared by dip coating has a support layer comprising regularly-sized pores, and an active layer having a dense structure with few pores. The composite membrane is limited in application, since the active layer has few pores of a size similar to those of the microfiltration or ultrafiltration membranes, and it is easily delaminated due to a weak interfacial adhesion strength between the support layer and the active layer.
The composite membrane may be prepared by coating a polymer on the microporous membrane as disclosed in U.S. Pat. Nos. 3,249,109, 4,388,189, and 5,102,552. In addition, a hydrophilic monomer, such as an acrylic acid, and polymers such as polyethylene oxide are grafted with corona treatment so that the membrane has a modified surface, and in particular so that it has hydrophilicity as disclosed in U.S. Pat. Nos. 4,346,142, 5,085,775, and 5,294,346. However, though the membrane has a modified surface and hydrophilicity, the method of graft polymerization is not applied, since the process is complicated and permeability of the membrane is not satisfactory.
A separator having regularly-sized pores for a common battery is coated with a polymer electrolyte solution, and it is used as a separator for a rechargeable lithium ion battery as disclosed in U.S. Pat. No. 5,716,421 and European Patent No. 0933824A2. However, when the separator is prepared by the aforementioned method, the membrane has a dense structure, that is, no pores are formed on the-surface of the membrane, and permeability (e.g. air permeability) deteriorates, and the interfacial adhesion strength between the support layer and the active layer is inadequate.